Over the past few years, a new category of data management products has emerged. They are variously called "object-oriented database systems", "extended database systems", or "database programming languages". They are intended to be used by applications that are generally complex, data-intensive programs, which operate on structurally complex databases containing large numbers of inter-connected objects.
Inter-object references, sometimes called pointers, provide this complex structure. These programs consume time by accessing and updating objects, and following the intricate connections between objects, using both associative queries and direct traversal, performing some amount of computation as each object is visited. Typical application areas are computer-aided design, manufacturing, and engineering, software development, electronic publishing, multimedia office automation, and geographical information systems. Because of this application environment it is important for an object-oriented database system to be fast.
Often, a number of work stations or other client computers are connected to access the database in a distributed manner, normally through a server computer associated with the database. Each client computer has its own cache memory in which data required by an application program being run on the client computer are placed.
Every object-oriented database system has some way to identify an object. Current systems use a thing called an "object identifier" (OID), which embodies a reference to an object. In a sense, an OID is the name of an object. An operation called "dereferencing", finds an object from a given name of an object.
In most systems, object identifiers are data structures defined by software, thus dereferencing involves a software procedure, such as a conditional test to determine whether the object is already in memory, which often involves a table lookup. This software procedure generally takes at least a few instructions, and thus requires a fair amount of time. Moreover, a dereferencing step is completed for each access to the object. These operations significantly slow down processing in an application, specifically when many inter-object references are made.
Moreover, names that are commonly used for object identifiers are not in the same format that the computer hardware uses as its own virtual memory addresses. Thus, inter-object references take longer to dereference than ordinary program data. Furthermore, a software conditional check takes extra time.
Also, in current systems, data cannot remain in the client computer between transactions. Data can be cached on the client computer, but when a transaction ends, the client cache has to be discarded. Although this requirement insures consistency of data, it increases communication between the client and the server computers and fails to make use of the principles of locality which encourage the use of a cache in the first place.
A need, therefore, exists for an improved method and apparatus for facilitating dereferencing the name of an object to its corresponding object.
Another object of the invention is to name objects using the format of the computer hardware. More particularly, it is an object to provide virtual addresses as pointers to objects in the database.
Another object of the invention is to provide a hardware conditional check for determining if an object is in virtual memory in order to replace software conditional checks.
Still another object of the present invention is to minimize communication between a server computer and a client computer. More particularly, it is an object to provide a mechanism to allow a client computer to keep data in its cache between transactions and to ensure data consistency and coherency.